>>3378937dunno, haven't seen them for sale online on their own, besides the ones on ebay taken off original lenses for stupid high prices.
ask your analog camera tech for the bunny ears conversion. they get the prong machined and bent from a piece of steel and install it for around $25.
or if you're crafty, you can get the piece machined at a local workshop for a few bucks and install it yourself. however, the existing guide "holes" on the lens are just marks, you need to carefully drill a little to get the screw to actually thread in, and you also need to get short screws that won't go past the aperture ring and hit the lens's body
pic related, you can see the guide marks between the larger and smaller f4 and f8 on the aperture ring